Electronic equipment is often powered from a DC power source, such as a battery or DC power supply, and the supplied DC voltage must be well filtered and regulated to be useful.
A DC-DC power converter is an electronic circuit that converts a source of direct current from one voltage level to another. DC-DC converters are widely used in a variety of electronic equipment applications, including power supplies for portable electronic devices, and are used generally in electronic circuits to regulate an output voltage. For example, a DC-DC converter may be used to compensate for varying load current and/or variations in the input voltage.
Linear regulators are commonly used and have many desirable characteristics, such as simplicity, low output ripple, excellent line and load regulation, fast response time to load or line changes, and low EMI. However, they also have low efficiency due to power dissipated as heat, and occupy large volumes of silicon real estate. Thus, linear regulators are practical only if the current requirement is low.
Switching mode regulators are generally preferred to linear regulators because the conversion method is more power efficient for many applications. In a switching mode converter, a power field effect transistor (“FET”) is used to switch efficiently at high frequency.
Pulse width modulation (“PWM”) is a converter architecture that uses a fixed-frequency oscillator to drive the power switches to thereby transfer energy from the input to the output. The oscillator drive signal is fixed in frequency but varies in duty cycle, namely the ratio of switch ON time to the total switching period. The duty cycle is varied by adjusting the pulse width of the drive signal for each clock cycle based on operating conditions.
PWM converters offer a predictable operating frequency, low output ripple characteristics, and high efficiency during moderate to heavy load conditions. However, PWM converters suffer from poor conversion efficiency at light load or standby conditions, i.e., when the demand for energy from the load is less than the energy that can be supplied by the regulator circuit.
Pulse frequency modulation (“PFM”) is an alternative converter architecture that uses a variable-frequency clock to drive the power switches. In particular, PFM control mode can provide better low-power conversion efficiency and simple converter topologies.
Thus, it would be desirable to provide a converter/regulator that utilizes PWM for normal load conditions and PFM for light load conditions, with automatic and seamless transition between the two modes.